1. Field of Invention
Aspects and embodiments disclosed herein are generally directed to electrochemical devices, and more specifically, to electrochlorination cells and devices, methods of fabricating same, and systems utilizing same.
2. Discussion of Related Art
Electrochemical devices based on chemical reactions at electrodes are widely used in industrial and municipal implementations. Examples of reactions include:
A. Electrochlorination with generation of sodium hypochlorite from sodium chloride and water.
Reaction at anode: 2Cl−→Cl2+2e−
Reaction at cathode: 2Na++2H2O+2e−→2NaOH+H2 
In solution: Cl2+2OH−→ClO−+Cl−+H2O
Overall reaction: NaCl+H2O→NaOCl+H2 
B. Generation of sodium hydroxide and chlorine from sodium chloride and water, with a cation exchange membrane separating the anode and the cathode:
Reaction at anode: 2Cl−→Cl2+2e−
Reaction at cathode: 2H2O+2e−→2OH−+H2 
Overall reaction: 2NaCl+2H2O→2NaOH+Cl2+H2 
C. Vanadium redox battery for energy storage, with a proton permeable membrane separating the electrodes:
During Charging:
Reaction at 1st electrode: V3++e−→V2+
Reaction at 2nd electrode: V4+→V5++e −
During Discharging:
Reaction at 1st electrode: V2+→V3++e−
Reaction at 2nd electrode: V5++e−→V4+
This disclosure describes various embodiments of electrochlorination cells and electrochlorination devices, however, this disclosure is not limited to electrochlorination cells or devices and the aspects and embodiments disclosed herein are applicable to electrolytic and electrochemical cells used for any one of multiple purposes.
Current commercially electrochlorination cells are typically based on one of two electrode arrangements, concentric tubes (CTE) and parallel plates (PPE).
FIGS. 1A and 1B show an example of an electrochlorination cell 100 with concentric tubes 102, 104 manufactured by Electrocatalytic Ltd. The inner surface of the outer tubes 102 and the outer surface of the inner tube 104 are the active electrode areas. The gap between the electrodes is approximately 3.5 mm. For marine and offshore applications with seawater as feed, the liquid velocity in the gap in the axial direction can be on the order of 2.1 m/s, resulting in highly turbulent flow which reduces the potential for fouling and scaling on the electrode surfaces.
FIGS. 2A-2C show some possible arrangement of electrodes in a CTE electrochemical cell. FIG. 2A illustrates an arrangement in which current flows in one pass from the anode to the cathode. Both electrodes are typically fabricated from titanium, with the anode coated with platinum or a mixed metal oxide (MMO). The electrodes are called “mono-polar.”
FIG. 2B illustrates an arrangement in which current flows in two passes through the device with two outer electrodes and one inner electrode. One of the outer electrodes is coated on the inside surface to serve as an anode; the other is uncoated. A portion of the outer surface of the inner electrode is coated, also to serve as an anode, and the remaining portion is uncoated. Current flows through the electrolyte from the coated outer electrode to the uncoated portion of the inner electrode, along the inner electrode to the coated portion, then finally back across the electrolyte to the uncoated outer electrode. The inner electrode is also called a “bipolar” electrode.
FIG. 2C illustrates an arrangement in which current flows in multiple passes through the device with multiple outer electrodes and one inner electrode. By alternating coated and uncoated outer electrodes and coating the inner electrodes at matching intervals, current can flow back and forth through the electrolyte in multiple passes.
The rationale behind multiple passes is that the overall electrode area available for electrochemical reaction at the surface, and therefore the overall production rate of disinfectant (e.g., sodium hypochlorite), can be increased without a proportional increase in applied current. Increasing the electrical current would require larger wires or bus bars from the DC power supply to the electrochlorination cell, larger electrical connectors on the cell (lugs on the outside surface of the outer electrode in the example in FIG. 1A) and thicker titanium for the electrodes.
For the same current, a multiple pass device will have higher production rate than a single pass cell but the overall voltage drop will be higher (approximately proportional to the number of passes). For the same production rate, a multiple pass cell will require lower current (approximately inversely proportional to the number of passes). For the same power output (kW), power supply costs may be more sensitive to output current than output voltage, thereby favoring the multi-pass cells.
In actuality there are inefficiencies associated with a multiple pass cell. For example, a portion of the current, referred to as “bypass current,” can flow directly from an anode to a cathode without crossing the electrolyte in the gap between the outer and inner electrodes (see FIGS. 2B and 2C). The bypass current consumes power but does not result in production of the disinfectant. Multiple pass cells are also more complex to fabricate and assemble. Portions of the outer surface of the inner electrode, for example, must be masked before the remaining portions are coated.
FIG. 3 shows a parallel plate electrochlorination (PPE) cell and FIG. 4 is a schematic of a multiple-pass unit with sets of flat electrodes arranged in parallel. The sets of electrodes at each end are electrically connected in parallel, with one set connected to a positive output from a DC power supply and other set connected to the negative output. The electrodes in between are bipolar. One advantage of the multiple pass parallel plate design vs. the concentric tubular design is the higher packing density of active electrode area per unit volume of the device, since both sides of each electrode are exposed to the electrolyte solution and therefore participate in electrode reactions. The tighter packing and multiple passes result in higher pressure drop in the PPE cell than in the CTE cell. The mean flow velocity between the plates can be reduced to lower the pressure drop and increase hydraulic residence time; the downside is increase in risk of fouling and scaling and therefore more frequent cleaning with acid, for example.
A frame structure is required in a PPE cell to mechanically support the multiple plates and maintain a specified spacing between adjacent electrodes. Electrical connection to multiple plates at each end may also be challenging.
In both CTE and PPE cells, removal of H2 gas generated at the cathodes is a major challenge in the design of the devices and of the overall system. The gas must be safely vented at either selected locations in the piping or at product tanks.